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unsolve problems Particle Physics - Progress, 1 of 3 progress page 2

As with most other branches of science, particle physics have seen rapid progress in recent years. The following lists, in chronological order, highlight some of these progresses. Some may be controversial and some probably need more secrutiny. Nevertheless, the list will be updated from time to time as more discoveries being made. Comments are welcome and will be considered to publish on

Muon's g-factor - February 2001
Precise measurement of muon's g-factor may have yielded new physics beyond the Standard Model. Since all leptons (as well as quarks) have intrinsic spin, they also have a magnetic moment, which is related to spin by the g-factor. According to quantum theory prediction, g = 2 for both electron and muon. However, in reality, radiative corrections will have to be considered in order to take into account the continuous adsorption and emission of short-live 'virtual particles' by the electron or muon that perturb the spin magnetic moment. These corrections depend very much on the existence of other particles, such as photons, electrons and also other particles yet undiscovered and are not part of the Standard Model. One possible candidate for the latter particles are superpartners, which according to the supersymmetry (SUSY) theory, that every fundamental particle has a companion particle called its superpartner. For example, the partner of muon is called 'smuon'. To date there has no firm evidence for supersymmetry.

For this reason, the g-factor for muon turns out to be about 2.0023. Instead of directly measuring g, particle phyicists usually measure the anomalous g-factor, a = (g-2)/2. For electron, this quantity determined experimentally is in aggreement with the Standard Model to 9 decimal places - indicating there is almost certainly no other new particle exists outside the Standard Model prediction. However, muon is more than 200 times heavier than the electron and its g-factor is 40,000 more sensitive than the electron to detect any new particles beyond the Standard Model. However, two main factors make g-factor measurement for muon difficult: larger radiative correction and inherently short half-life (2 microseconds). But now, scientists from the Brookhaven National Laboratory has obtained a value of a = 11659202 x 10-10. This differs from the Standard Model prediction by 2.6 standard deviation. Statistically, this means there is a 99% probability that the measurement does not agree with the Standard Model. If data are confirmed not to subject to any systematic error, could SUSY being the most plausible explanation?

CP violation in B-mesons - July 2001
The present Universe contains no celestial object made of antimatter. It is though that equal amount of matter and antimatter was created during the early stage of the Big Bang, which when a matter and an antimatter encounter each other will be annihilated, giving out photons. However, through a process called charge-parity (CP) violation, our universe is entirely made up of small excess of matter that remained after annihilation. In fact, CP violation was first observed in 1964 in difference in decay properties of neutral kaon-mesons, K0, and its antiparticle. It is also though that the heavier neutral B-mesons, B0, may show a similar effect.

The amount of violation, as predicted by the Standard Model is to describe in terms of the angular quantity sin 2b: if there is no violation, then sin 2b = 0. The angular quantity is required in order to calculate the 'unitarity triangle' which area quantifies the amount of violation. In the early July 2001, the CP violation in the B-mesons is discovered by scientists from BaBar that report sin 2b = 0.59 +/- 0.14, and later in the month, an experiment from the KEK laboratory in Japan found sin 2b = 0.99 +/- 0.14. Both teams found that the B-mesons decay slightly slower than their antiparticles.

A constant that changes with time? - August 2001
The fine-structure, a = 2pe2/hc, was found to change with time (Phys. Rev. Lett. 87 091301 (2001)). The fine structure constant is the dimensionless number (0.007297...) that determines the strength of interactions between charged particles and electromagnetic fields and is related to splitting in the emission and adsorption spectroscopy. It was found that the contant has actually increased very slightly with time. This discovery was derived from several independent sources of measurement of the adsorption spectra of distant quasars at different redshifts and then compare the wavelengths of certain present days spectral lines. If the constant does indeed change with time, then at least one of the fundamental constants e, the electric charge, h, Planck constant, and/or c, the speed of light, may have also change with time. Further confirmation measurements obtain from other sources need to be carried out. So far, no evidence has ever been found for these physical constants to change, although the behavior is predicted in certain unified theories of fundamental forces.

The quantum effects of gravity - December 2001
An experiment shew that neutrons falling through a gravitaional field jump from one height to another, instead of doing so continuously (Nature 297, 415 (2001)). The discrete nature is the prediction of quantum theory. The work was carried out by bouncing ultra-cold neutrons off a horizontal mirror. Neutrons were used because they have zero charge and long lifetime. These characteristics minimize effects of other forces, leaving only the gravitaional field being the major force. It was found that as neutrons were bounced off the mirror, the minimum height they attain was about 15 mm. In other words, no neutron was detected below this height and there was a sudden jump in the neutron count at this height and smaller jumps at a several other heights.

According to the report, these heights correspond to peaks in a standing wave created by the neutron's de Broglie wave interfering with itself when reflected by the mirror. The first peak agreed with the theory, but other weaker peaks still need to be confirmed by scientists. The work is not directly related to the detection of elusive quantum theory of gravity because the field itself is not quantized. However, the experiment could be used to study the reason for the equal accleration of all matter in a gravitaional field.

unsolve problems progress page 2


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